1. Field of the Invention
The subject invention relates to electrooptical systems, to light gating methods and apparatus, to electrode systems, to technique for increasing feasible electrode and light gate densities, to methods for inhibiting crosstalk or spurious cross-energization between light gates, to features for inhibiting photo-induced birefringence, to techniques for reducing or eliminating electrode degradation, and to methods and apparatus for processing, displaying, recording, writing and reading information.
2. Disclosure Statement
This disclosure statement is made pursuant to the duty of disclosure imposed by law and formulated in 37 CFR 1.56(a). No representation is hereby made that information thus disclosed in fact constitutes prior art, inasmuch as 37 CFR 1.56(a) relies on a materiality concept which depends on uncertain and inevitably subjective elements of substantial likelihood and reasonableness, and inasmuch as a growing attitude appears to require citation of material which might lead to a discovery of pertinent material though not necessarily being of itself pertinent. Also the following comments contain conclusions and observations which have only been drawn or become apparent after conception of the subject invention or which contrast the subject invention or its merits against the background of developments subsequent in time or priority.
Various electrooptical light gate systems have been proposed in diverse fields of utility. By way of example, reference may be had in this respect to an article by J. Thomas Cutchen et al, entitled Electrooptic Devices Utilizing Quadratic PLZT Ceramic Elements, published in 1973 WESCON TECHNICAL PAPERS, Vol. 17, part 30, pp. 30/2 et seq. These and other articles in the field describe electrooptic ceramics and devices employing transparent lanthanum-modified lead zirconate titanate (PLZT), and application thereof, including page composers, display devices, eye protective devices, industrial welding protection, large aperture photographic shutters and variable density filters.
Facsimile apparatus for writing and reading mechanically moving documents with an electronically controlable switching mask plate, disposed between polarization filters and consisting of a material containing mixed crystals of lead zirconate and lead titanate, doped with lanthanum, and provided with aligned electrodes, were proposed in U.S. Pat. No. 3,930,119, by Rolf Schmidt et al, issued Dec. 30, 1950.
That proposal contemplated use of a switching mask plate having a large number of electrodes arranged in line with one another, or then employment of a plurality of ceramic plates provided with electrodes and stacked so that light beams could be masked according to a raster between the electrodes of the individual plates.
A further proposal is apparent from British Patent Specification No. 1 534 027, by Battelle Memorial Institute, published Nov. 29, 1978. That proposal employs light modulating elements which are arranged side by side, and each of which has the form of an electrooptical shutter. Electrode arrangements are also shown which have comb-like electrode configurations or similar arrangements with a grounded serpentine common electrode located on both major sides of a ceramic plate, with spatial coincidence or registration existing among corresponding electrodes on the two substrate faces.
As representative, though by no means exhaustive, of the vast literature on the subject, reference may also be had to an article by Kei-ichi Ueno and Tadashi Saku, entitled PLZT Spatial Light Modulator for a 1-D Hologram Memory, published in APPLIED OPTICS, Vol. 19, No. 1 (Jan. 1, 1980), and to two papers published by THE BRITISH BROADCASTING CORPORATION (BBC), Research Department, Engineering Division. One of these BBC papers, by K. Hacking and I. Childs, is entitled Digital Recording Using Hologram Arrays: Laser-Beam Deflection and Modulation, BBC RD 1979/6, UDC 621.375.9; 535.8 (March 1979). The other of these BBC papers, by K. Hacking and J. L. Riley, is entitled Digital Recording Using Hologram Arrays: Development of the Transducers for an Experimental 100 M bit/s Recorder, BBC RD 1979/16, UDC 621.375.9; 535.8. This literature starting with the above mentioned 1973 WESCON Technical Papers, expresses considerable concern about crosstalk between light gates of electrooptic devices. In this respect, "crosstalk" is a term of art borrowed from the field of telephony. Since no talk is typically involved in electrooptical light gate systems, the expression "spurious cross-energization" is sometimes used herein instead.
By way of background, the above mentioned 1973 WESCON article, on page 10, deals with the problem of crosstalk and indicates that such problem inhibits a close packing of light gates. To overcome the problem, the particular article recommends a spacing of gates, in a single-aperture structure, on 0.010-inch (0.25 mm) centers. In practice, this imposes a severe limitation on attainable gate density. For the more easily driven dual aperture gate structure, the indicated spacing for crosstalk reduction would even have to be doubled, which would reduce attainable gate density still more.
From comparion, crosstalk may be considered in terms of light transmission in neighboring gates of a given open light gate. In this respect, it has been found in a dual gate structure having a density of 80 gates per inch, with two subgates per gate, that light transmission in a first neighboring gate was still about a fiftieth, in a second neighboring gate about a two-hundredthirtieth, and in a third neighboring gate about a two-hundred-fiftieth of the light transmission in the open reference gate. In a dual gate structure having a density of 256 light gates per inch, light transmissions of a fiftieth, a one-hundred-twenty-fifth, a two-hundred-thirtieth and a two-hundred-fiftieth were determined as taking place in the first, second, third and fourth neighboring gates, respectively, relative to the light transmission through the open reference gate.
Considerable crosstalk reductions were observed from employment of a technique suggested in the above mentioned BBC papers. In this respect, the cited paper by K. Hacking and I. Childs suggests on page 31 that PLZT material be provided outside of the light gates with a layer having a dielectric constant significantly smaller than the relatively high dielectric constant of PLZT material, and that terminals and leads for the light gates be positioned on such dielectric layer in order to reduce crosstalk between adjacent gates.
Using that kind of dielectric layer technique in the above mentioned 80 per inch density gate structure, the spurious light transmission was only a two-hundred-thirtieth in the first neighboring gate, a two-hundred-fiftieth in the second neighboring gate and a three-hundredth in the third neighboring gate, relative to the reference gate. Similarly with the mentioned dielectric layer technique, spurious light transmission in the above mentioned 256 per inch gate density structure was somewhat reduced to about an eightieth, a one-hundred-seventy-sixth and a hundredthirtieth in the first, second and third neighboring gates, respectively, relative to the reference gate.
While these improvements are remarkable, they do not go far enough for some applications and they address themselves more to the light gate terminal structure than to the light gates themselves.
That proposals from other fields offer no solution to the above mentioned problems may, for instance, be seen from a consideration of U.S. Pat. No. 3,124,635, by E. M. Jones et al, issued Mar. 10, 1964, and proposing several electrode structures for photoelectric musical instruments and the like, U.S. Pat. No. 3,452,342, by J. I. Raffel, issued June 24, 1969, and proposing strip conductor designs for high-capacity memory circuit arrangements, and British Patent Specification No. 1 411 846, by Michael N. Ernstoff et al, published Oct. 29, 1975, and disclosing electronically switched field sequential color television systems with interdigitated electrode structures.
References cited in the above mentioned literature and patent papers, as well as references cited elsewhere herein, may also be considered in this context.
Another problem that has so far beset electrooptical light gate systems, such as PLZT light shutters or gate arrays, may be designated as "photoinduced birefringence." Such undesired birefringence occurs, for instance, when light gates are illuminated at high intensity while electric fields are applied to the gates, such as to place the gates in their ON condition. In short, photoinduced birefringence degrades the performance of light gates, and particularly their light transmitting capability.
While no limitation to or dependence on any particular theory is intended, it may be observed that a cause of photoinduced birefringence is believed to be photoexcitation of electrons in the illuminated region where electrons can reach the conduction state and drift under the influence of the applied field. Such photoinduced charge carriers are being trapped in the dark regions of the electrooptical material layer; particularly in close proximity to the edges of the electrode deposits. In brief, the trapped photoinduced charge carriers generate a space charge field which acts against the field applied to the electrodes and gating regions. The localized charge at the boundary of the illuminated area grows at a rate proportionate to the difference of free carrier concentration in the illuminated and in the dark areas, which in turn is dependent on the light and field intensities. By way of example, up to 45% decrease in light transmission was observed with PLZT light gate arrays after 90 minutes of intense illumination.
Especially with PLZT light gate systems, the rule of thumb has developed that the interelectrode spacing, that is, the spacing between immediately adjacent electrodes, should not be smaller than the thickness of the layer of electrooptically active material. Such relationship, in effect, provides a favorable distribution of the applied electric fields throughout the thickness of the electrooptically active material, thereby reducing the above mentioned light induced birefringence effect. However, the latter rule of thumb places severe restrictions on a reduction of interelectrode spacing for a given thickness of the electrooptically active substrate or layer. As a result, existing systems are either subject to light induced birefringence at a detrimental rate or suffer a severe limitation on the attainment of desirable properties, such as an achievement of high resolution.
Prior proposals, such as those contained in the above mentioned Schmidt et al patent, in U.S. Pat. No. 3,799,647, by Victor Luft, in British Patent Specification No. 1 465 673, by R. W. Cooper, incidentally showing illumination through the side of an electrooptical layer opposite the electroded side, and in an article by W. F. Gilmore, entitled "Printed Circuit Generator," IBM Technical Disclosure Bulletin, Vol. 12, No. 7 (December 1969), showing illumination through the length or width of an electrooptical crystal, offer no solution of this serious problem.
Similarly, even though angled profiles have been used in breadboard structures since vacuum tube radio days, as may be seen from an advertisement for "ZAK Breadboards," 1955-03 (March, 1955) pp. 3-7, the prior art has been attempting to bond electrooptical light gate materials with substrates having different coefficients of expansion and other characteristics, thereby inducing strain in the electrooptical light gate material and otherwise impeding the proper operation of the desired light gating function.
Moreover, even though different electrode widths and spacings have been proposed in the past, including wider, as well as narrower gaps than the electrode width, the prior art most likely used equal electrode widths and spacings or did at least not show any particular preference either way. Reference may in this respect be had to the above mentioned 1973 WESCON Technical Papers, such as pp. 9 and 10, and to the article "PLZT Electrooptic Shutters: Applications," by J. Thomas Cutchen, et al, APPLIED OPTICS, Vol. 14, No. 8 (August 1975), such as pp. 1871 and 1872, describing linear array page composers having a gap width smaller than the electrode width, as well as having equal gap and electrode widths.
The above mentioned Hacking et al paper, BBC RD 1979/6 also deals with the subject of design of electrodes such as on pages 24 to 26 thereof, commenting that for a smaller gap dimension, the width of the electrodes should be at least twice the gap dimension to insure maximum field at the center of the gap for a given applied voltage, and commenting further that, in practice, the corners of the electrodes are rounded thus reducing the area of uniform field inside the PLZT.
Closer examination of these statements, particularly in the light of FIG. 19 of the particular BBC paper, shows that that paper is referring to an electrode arrangement in which the light gate gap is located between smaller electrode ends, with the electrodes themselves being aligned with their longitudinal dimensions along an axis traversing the light gate gap and with the electrode thus facing each other with their small ends across such light gate gap. Reference may also be had to the article "A Linear Electrooptic Effect in Ferroelectric Ceramics: PLZT 12/40/60," by Philip D. Thacher, FERROELECTRICS 1972, Vol. 3, pp. 147-150, containing analyses of field distribution with spaced electrodes on PLZT wafers.
Despite such prior work, no comment on the subject of electrode and light gate degradation was apparent, and it appears that such a problem either was ignored or not even recognized by the prior art.